Formation and evolution of Archean continental crust: A thermodynamic – Geochemical perspective of granitoids from the Tarim Craton, NW China

When and how continental crust formed and evolved to its current state is a fundamental question regarding crust - mantle evolution and geodynamic regimes on the early Earth. Earth's Archean continental crust is dominated by sodic granitoids comprising tonalite - trondhjemite - granodiorite (TTG), whereas diverse high-K granitoids appear relatively late in the geological record. Therefore, the formation and secular evolution of Archean TTGs and high-K granitoids is key to many issues regarding early Earth geodynamics. The Tarim Craton, NW China, is an ancient continental block containing some of the oldest rocks (-3.7 Ga TTGs) on Earth, but its Archean evolution is poorly known due to limited outcrop. Here we review the Archean geological record of the Tarim Craton and present the new findings of -3.2-3.0 Ga TTG, -2.8 Ga high-K granite and -2.35 Ga A-type granitoids (mostly syenogranite and syenite) in the southwestern Tarim. The -3.2-3.0 Ga TTG gneisses have homogenous and highly radiogenic zircon Hf isotopic compositions (weighted mean epsilon Hf = +3.5 to +4.4), indicating crustal growth from a long-term depleted mantle. In contrast, the -2.8 Ga high-K granite and -2.35 Ga A-type granitoids have lower epsilon Hf values (weighted mean + 2.0 and - 4.3 to -7.8, respectively) that plot on the evolution trend of the Mesoarchean juvenile crust, indicating repeated crustal reworking and progressive crustal differentiation. Thermodynamic - geochemical modelling demonstrates that water-fluxed melting is an efficient way to produce large volumes of TTG melts, and that melting pressure plays a primary role in determining the trace element systematics of diverse TTGs. Water-fluxed melting at crustal (10-12 kbar) and mantle (18-20 kbar) depths explains not only the compositions of the -3.2-3.0 and -3.7 Ga TTGs in the Tarim Craton, respectively, but also the global average compositions of median- to low-pressure and high-pressure TTGs. Partial melting of the Mesoarchean gneisses at shallow depth (<= 5 kbar) reproduces the trace element patterns of the -2.35 Ga A-type granitoids in SW Tarim, but cannot explain the low SiO2 of the syenites, which probably originated from partial melting of an enriched mantle resulting from recycling of Archean continental crust during continental rifting. This process also contributed to the diversification of granitoids and maturation of continental crust. Based on these results, we propose a new model for the formation and evolution of Archean continental crust. In our model, most Archean TTGs were produced by water-fluxed melting of thick proto-arcs built on thick Archean oceanic crust, whereas rare high-pressure TTGs were formed by water-fluxed melting of subducted arcs during intermittent subduction and arc accretion. Our model links the origin of Earth's continental crust to an early hydrosphere and subduction - accretion tectonics, and explains the decline of TTGs and rise of high-K granitoids by secular cooling of the mantle, which resulted in thinner oceanic crust and arcs, facilitating arc subduction, rather than arc accretion, and enhancing the rate of crustal reworking relative to crustal growth.